Light Emitting Diodes (LEDs) are semiconductors that emit light when electrical current flows through them. As LED technology has advanced, LED-based lighting applications have moved from indicators and control displays to general lighting apparatus, such as those that would be used in a home to illuminate a room, or in other applications requiring fairly intense light, such as automobile headlamps. The use of LEDs in the home is advantageous over conventional lighting devices—such as incandescent bulbs, fluorescent lighting, and compact fluorescent bulbs—because LEDs tend to use less electricity than the aforementioned conventional lighting devices while providing similarly intense light. Furthermore, under normal operation, LEDs, on average, tend to last longer than conventional lighting devices.
However, before LEDs gain widespread acceptance for residential or commercial applications, lighting devices employing LEDs must provide at least the same feature set as conventional lighting devices. One feature that conventional LED systems currently lack is the ability to allow a lamp employing an LED to utilize the full functionality of a dimmer switch (or dimmer).
LEDs typically operate on direct current (DC). Accordingly, incorporating LEDs into residential and commercial lighting systems (which are conventionally wired to receive alternating current (AC)) requires that the LED be connected to a “driver” circuit that converts alternating current (for example, the common United States residential supply is 120V at 60 Hz in the United States) into an appropriate direct current for the LED. Conventional LED driver circuits, when used with a conventional dimmer, are generally not capable of dimming an LED below 20% of its full intensity. Specifically, in conventional circuits, when the dimmer is positioned such that a LED coupled thereto should be reduced to its lowest possible level of illumination, the LED is only reduced to approximately 20% of its full intensity.
One solution for dimming an LED to levels below 20% of full intensity is to utilize pulse width modulation (PWM). Using PWM, pulses of current of varying widths are driven through the LED. As the width of the pulses decrease, the total power delivered to the LED decreases, causing the LED to dim. The pulses can then be applied at a high enough frequency such that humans cannot detect any flicker in the light output by the LED. The use of PWM to dim LEDs has several drawbacks, however, especially with respect to residential or commercial lighting devices. First, PWM is expensive to implement, requiring additional circuitry to generate and modulate the pulses. Second, PWM control circuitry is much more sophisticated than conventional circuits, and accordingly, the engineering time required to design and implement PWM to dim LED circuits is much higher than conventional dimmers.
Accordingly, a need exists in the art for a simple, inexpensive LED driver that allows an LED to be dimmed to levels below 20% of full intensity.